Vegetable oils are typically oil that is pressed or extracted from vegetable sources. Almost every vegetable oil contains some form of phosphatides (hydratable or non-hydratable), commonly known as gums. Soybean oil contains about 1-3%, corn oil 0.6-0.9%, sunflower oil 0.5-0.9%, and canola oil (crude) 1-3% of phospholipids.
The main components to be removed during vegetable oil refining are the free fatty acids (FFAs) and phospholipids. They are usually removed by applying an acid treatment and caustic soda (alkali) treatment in a neutralization step. The concentration and amount of the alkali to be used will vary with the free fatty acid (FFA) content of the oil.
Neutralization is an important step in the chemical refining of vegetable oils. Alkali neutralization causes a major loss of neutral oil in the chemical refining of edible oils.
Oil loss occurs in two ways; saponification loss and separation loss. Saponification loss is the consequence of alkali being in contact with neutral oil. While alkali is much more reactive with free fatty acid saponification and phosphatide hydration, some reaction occurs with the triglycerides (hydrolysis and subsequent saponification of the free fatty acids). By keeping the alkali strength low, and reducing its contact time and the contact temperature with the oil, saponification of triglycerides can be minimized.
Another occurrence of oil loss can be in the separation of the refined oil (light phase) from the soapstock (heavy phase) produced in alkali refining. Free fatty acids (FFA) are generally removed in neutralization as sodium soaps but neutral oil is also entrapped in the emulsion and removed with the soap during centrifugation. To achieve the lowest separation loss, the process design should provide minimum product stream passing through a centrifuge.
Some improvements have been introduced in oil treatment processes. Improved mixing of chemicals in caustic soda and acid treatment using ultra high shear mixers with regard to finely dispersing the acid/base solution in the oil, and the introduction of enzymes (phospholipases) to specifically attack the gums, have substantially improved the efficiency and oil yields in vegetable oil refining processes.
A method disclosed in U.S. Pat. No. 4,240,972 includes adding an acid to a heated stream of crude vegetable oil and then immediately passing the mixture through a static mixer, mixing for a fraction of a second to produce an acid-in-oil dispersion having acid droplets smaller than 10 microns, and then separating the dispersion into an oil phase and an aqueous phase containing the phosphatides.
U.S. Pat. Nos. 4,698,185 and 6,015,915 describe processes for degumming vegetable oil using high shear Ultra-Turax rotor/stator apparatus.
U.S. Pat. No. 6,172,248 describes improved methods for refining vegetable oils and by-products thereof. In an organic acid refining process, vegetable oil is combined with a dilute aqueous organic acid solution and subjected to high shear to finely disperse the acid solution in the oil. The high shear mixing can include an impeller operating at conditions that produce flow velocities of at least about 45 feet per second. High shear mixing according to the patent generally requires at least one impeller rotating at a speed of from about 900 to about 1500 rpm and having a blade tip speed of from about 4000 to about 9000 ft/min, thereby generating high shear flow velocities of at least about 45 feet per second.
U.S. Pat. No. 6,844,458 describes a process that mixes the acid-oil blend at high shear for a time sufficient to finely disperse the organic acid in the vegetable oil and the high shear mixing occurs for a time of less than about 30 seconds.
U.S. Pat. No. 8,491,856 describes a system for stripping fatty acids from triglycerides with the use of a high shear device with at least one rotor, and wherein the at least one rotor is rotated at a tip speed of at least 22.9 m/s (4,500 ft/min) during formation of the dispersion. The energy expenditure of the high shear device may be greater than 1000 W/m3 during formation of the dispersion. The high shear device includes at least one stator and at least one rotor separated by a clearance.
U.S. Pat. Pub. No. 2009/0306419 describes a method for degumming triglyceride oils by subjecting the feed stream to high shear conditions having a shear rate of at least about 5,000 sec−1 and residence times of about 0.05 to 0.5 second.
Allowing the alkali solution and the crude or degummed oil to remain in contact with one another for only short periods of time can minimize saponification losses but it can be insufficient to remove impurities other than fatty acids, especially impurities such as phosphatides and metal compounds.
Accordingly, there is a continuing need for alternative refining methods, which can provide cost-effective removal of phosphorous, with improved oil yield.